This invention relates to well casing hangers, and more particularly to adjustable mandrel casing hangers for use with mudline tie-back apparatus.
One of the procedures of the petroleum industry to complete wells drilled at offshore locations is to use a tie-back sub or string to connect the upper end of the casing at the mudline to a surface-located wellhead. Because of the difficulty in spacing out long tie-back casing strings the surface casing hanger assembly must be able to adjust up or down a substantial distance, for example, forty-eight inches, be able to support a large tension load weight of, for example, 300,000 pounds in addition to the casing string weight, and maintain a reasonable height of the surface wellhead without having to machine the excess casing or hanger to the correct length.
Prior to the present invention three different methods were employed to overcome these subsea tie-back problems. The first was to adjust the casing hanger up or down, and then cut the hanger or casing to the proper length. Although this method works, it is very expensive due to the amount of rig time that it wastes. The second method was to make the surface wellhead large enough to swallow the entire hanger or casing plus the packoff. This works well for small adjustments, such as two to four inches, but becomes very cost prohibitive and space limiting when the needed adjustment grows to forty-eight inches. The third method was to move the entire wellhead up or down as required, but this is very expensive and requires the rig to have unlimited height capability, so that this method is not very effective.